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What Causes a Car to Stall?

You trust your car. You may not think about it, but you believe that certain functions will occur without fail. You may be aware of little annoyances or issues, but you trust that when you start your car, you will be able to drive to your destination and turn the ignition off until you are ready to drive again.

When your engine stalls, immediately that blind trust you have in your vehicle disappears. Will it happen again? What caused it? Do you need to get it looked at? Is it still safe to drive? Suddenly, you question your vehicle’s reliability. What caused your car to stall?

There are a number of vehicle problems that can cause stalling. Here are just a sample of the most common issues.

The fuel system

Whether it is a fuel quality issue or a fuel supply or fuel injector issue, the fuel system can be the cause of your car stalling. Three things from the fuel system can cause your engine to stall: bad fuel, too little fuel, and too much fuel.

When poor fuel quality is the problem, your engine’s combustion process isn’t efficient. If there are contaminants or water in the fuel, a misfire can occur. That is when the spark plug isn’t able to fire or the fuel in the engine isn’t able to burn. It can cause a stumble, or can cause the engine to stall if it happens consistently.

Your engine can stall if it is being starved of fuel. If the fuel pump isn’t supplying enough fuel to the engine, or if the air/fuel mixture is too lean, the engine doesn’t produce enough combustion in the power cycle to stay running, and will stall. A faulty fuel pump or incorrect timing can cause this.

If the engine is over-fuelling, that can cause it to stall. More commonly known as flooding, over-fuelling wets the spark plugs and they aren’t able to ignite the fuel in the cylinder. It causes a misfire that can cause the engine to stall. Faulty fuel injectors, and bad timing can cause over-fuelling.

Incorrect timing

Closely related to the fuel system is engine timing. A timing belt or chain synchronizes the different parts of the engine so it can run efficiently and properly. Many modern engine systems incorporate variable valve timing to optimize fuel efficiency and power production. If the variable valve timing is incorrect, it can cause the engine to stall from over-fuelling or starving the engine of gas.

If the timing belt or chain skips a tooth or more, the engine can and often does stall. The top end of the engine doesn’t turn in time with the bottom end. This can cause the engine not only to stall, but can cause internal engine damage in some cases.

Improper air supply

Air is as important to an engine as fuel. Without air, the engine simply will not run. In fact, air may be even more important since for proper combustion an engine needs 15 parts air to 1 part gasoline. Clean air is pulled through an air filter, then is metered into the engine by way of either a MAP (Manifold Pressure) sensor or MAF (Manifold airflow) sensor. Either way, if the engine doesn’t get the proper air supply, it will run rich and over-fuel. If the air supply is too high for the fuel introduced, it runs too lean. Both conditions can cause the engine to stall.

Electrical problems

Modern vehicles are equipped with modules that control everything from the heater and air conditioning to engine control. Vehicles can be equipped with dozens of different computers and modules. Communication between these modules is done by way of wiring, with a controller area network busline, or Canbus, being most common. As it is highly advanced technologically, any one of several wires or modules can fail and cause a lack of communication. The engine can stall because systems aren’t aware of what each other are doing.

A trained technician needs to inspect and diagnose these electrical concerns as specialized computers and tools are required to diagnose communication problems.

Transmission problems

You may not consider the possibility that your transmission can cause your engine to stall. However, it is possible. A transmission engaged in gear can sit still at a stop because the torque converter is allowing it to slip. If there is an internal pressure problem in the transmission, the torque converter may lock up and cause your vehicle to stall at an idle. Although not as common as other stalling conditions, it is a possibility.